diff --git a/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp b/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
index 0a4227c38224..c0355aef0b35 100644
--- a/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
+++ b/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
@@ -110,7 +110,8 @@ MCDisassembler::DecodeStatus WebAssemblyDisassembler::getInstruction(
       MI.addOperand(MCOperand::createReg(Reg));
       break;
     }
-    case WebAssembly::OPERAND_FPIMM: {
+    case WebAssembly::OPERAND_FP32IMM:
+    case WebAssembly::OPERAND_FP64IMM: {
       // TODO: MC converts all floating point immediate operands to double.
       // This is fine for numeric values, but may cause NaNs to change bits.
       if (Pos + sizeof(uint64_t) > Bytes.size())
diff --git a/llvm/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp b/llvm/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
index 2ecf9be23987..a58796c1ea82 100644
--- a/llvm/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
+++ b/llvm/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
@@ -110,14 +110,22 @@ void WebAssemblyInstPrinter::printInst(const MCInst *MI, raw_ostream &OS,
 }
 
 static std::string toString(const APFloat &FP) {
+  // Print NaNs with custom payloads specially.
+  if (FP.isNaN() &&
+      !FP.bitwiseIsEqual(APFloat::getQNaN(FP.getSemantics())) &&
+      !FP.bitwiseIsEqual(APFloat::getQNaN(FP.getSemantics(), /*Negative=*/true))) {
+    APInt AI = FP.bitcastToAPInt();
+    return
+        std::string(AI.isNegative() ? "-" : "") + "nan:0x" +
+        utohexstr(AI.getZExtValue() &
+                  (AI.getBitWidth() == 32 ? INT64_C(0x007fffff) :
+                                            INT64_C(0x000fffffffffffff)),
+                  /*LowerCase=*/true);
+  }
+
+  // Use C99's hexadecimal floating-point representation.
   static const size_t BufBytes = 128;
   char buf[BufBytes];
-  if (FP.isNaN())
-    assert((FP.bitwiseIsEqual(APFloat::getQNaN(FP.getSemantics())) ||
-            FP.bitwiseIsEqual(
-                APFloat::getQNaN(FP.getSemantics(), /*Negative=*/true))) &&
-           "convertToHexString handles neither SNaN nor NaN payloads");
-  // Use C99's hexadecimal floating-point representation.
   auto Written = FP.convertToHexString(
       buf, /*hexDigits=*/0, /*upperCase=*/false, APFloat::rmNearestTiesToEven);
   (void)Written;
@@ -157,10 +165,20 @@ void WebAssemblyInstPrinter::printOperand(const MCInst *MI, unsigned OpNo,
     // control flow stack, and it may be nice to pretty-print.
     O << Op.getImm();
   } else if (Op.isFPImm()) {
-    assert((OpNo < MII.get(MI->getOpcode()).getNumOperands() ||
-            MII.get(MI->getOpcode()).TSFlags == 0) &&
+    const MCInstrDesc &Desc = MII.get(MI->getOpcode());
+    assert(OpNo < Desc.getNumOperands() &&
+           "Unexpected floating-point immediate as a non-fixed operand");
+    assert(Desc.TSFlags == 0 &&
            "WebAssembly variable_ops floating point ops don't use TSFlags");
-    O << toString(APFloat(Op.getFPImm()));
+    const MCOperandInfo &Info = Desc.OpInfo[OpNo];
+    if (Info.OperandType == WebAssembly::OPERAND_FP32IMM) {
+      // TODO: MC converts all floating point immediate operands to double.
+      // This is fine for numeric values, but may cause NaNs to change bits.
+      O << toString(APFloat(float(Op.getFPImm())));
+    } else {
+      assert(Info.OperandType == WebAssembly::OPERAND_FP64IMM);
+      O << toString(APFloat(Op.getFPImm()));
+    }
   } else {
     assert((OpNo < MII.get(MI->getOpcode()).getNumOperands() ||
             (MII.get(MI->getOpcode()).TSFlags &
diff --git a/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCTargetDesc.h b/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCTargetDesc.h
index af970929bfaf..001bd7f1fc43 100644
--- a/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCTargetDesc.h
+++ b/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCTargetDesc.h
@@ -44,8 +44,10 @@ namespace WebAssembly {
 enum OperandType {
   /// Basic block label in a branch construct.
   OPERAND_BASIC_BLOCK = MCOI::OPERAND_FIRST_TARGET,
-  /// Floating-point immediate.
-  OPERAND_FPIMM,
+  /// 32-bit floating-point immediates.
+  OPERAND_FP32IMM,
+  /// 64-bit floating-point immediates.
+  OPERAND_FP64IMM,
   /// p2align immediate for load and store address alignment.
   OPERAND_P2ALIGN
 };
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.td b/llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.td
index 60e5d6ea6457..115e532b5a25 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.td
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.td
@@ -71,10 +71,11 @@ let OperandNamespace = "WebAssembly" in {
 let OperandType = "OPERAND_BASIC_BLOCK" in
 def bb_op : Operand<OtherVT>;
 
-let OperandType = "OPERAND_FPIMM" in {
+let OperandType = "OPERAND_FP32IMM" in
 def f32imm_op : Operand<f32>;
+
+let OperandType = "OPERAND_FP64IMM" in
 def f64imm_op : Operand<f64>;
-} // OperandType = "OPERAND_FPIMM"
 
 let OperandType = "OPERAND_P2ALIGN" in {
 def P2Align : Operand<i32> {
diff --git a/llvm/test/CodeGen/WebAssembly/immediates.ll b/llvm/test/CodeGen/WebAssembly/immediates.ll
index 735b386b4fc0..0ccfe0916f7a 100644
--- a/llvm/test/CodeGen/WebAssembly/immediates.ll
+++ b/llvm/test/CodeGen/WebAssembly/immediates.ll
@@ -133,6 +133,25 @@ define float @neginf_f32() {
   ret float 0xFFF0000000000000
 }
 
+; CHECK-LABEL: custom_nan_f32:
+; CHECK-NEXT: .result f32{{$}}
+; CHECK-NEXT: f32.const $push[[NUM:[0-9]+]]=, -nan:0x6bcdef{{$}}
+; CHECK-NEXT: return $pop[[NUM]]{{$}}
+define float @custom_nan_f32() {
+  ret float 0xFFFD79BDE0000000
+}
+
+; TODO: LLVM's MC layer stores f32 operands as host doubles, requiring a
+; conversion, so the bits of the NaN are not fully preserved.
+
+; CHECK-LABEL: custom_nans_f32:
+; CHECK-NEXT: .result f32{{$}}
+; CHECK-NEXT: f32.const $push[[NUM:[0-9]+]]=, -nan:0x6bcdef{{$}}
+; CHECK-NEXT: return $pop[[NUM]]{{$}}
+define float @custom_nans_f32() {
+  ret float 0xFFF579BDE0000000
+}
+
 ; CHECK-LABEL: negzero_f64:
 ; CHECK-NEXT: .result f64{{$}}
 ; CHECK-NEXT: f64.const $push[[NUM:[0-9]+]]=, -0x0p0{{$}}
@@ -196,3 +215,19 @@ define double @inf_f64() {
 define double @neginf_f64() {
   ret double 0xFFF0000000000000
 }
+
+; CHECK-LABEL: custom_nan_f64:
+; CHECK-NEXT: .result f64{{$}}
+; CHECK-NEXT: f64.const $push[[NUM:[0-9]+]]=, -nan:0xabcdef0123456{{$}}
+; CHECK-NEXT: return $pop[[NUM]]{{$}}
+define double @custom_nan_f64() {
+  ret double 0xFFFABCDEF0123456
+}
+
+; CHECK-LABEL: custom_nans_f64:
+; CHECK-NEXT: .result f64{{$}}
+; CHECK-NEXT: f64.const $push[[NUM:[0-9]+]]=, -nan:0x2bcdef0123456{{$}}
+; CHECK-NEXT: return $pop[[NUM]]{{$}}
+define double @custom_nans_f64() {
+  ret double 0xFFF2BCDEF0123456
+}